IMac freezes on start

Around a week ago my iMac, which is usually always on, would not wake from sleep (screen was black) and I had to hard reset it. However, it could not start up. After the apple logo appears on the grey screen and the tiny circle of dots flash to indicate that is loading, the 'dot circle' suddenly froze. The only option was again to hard reset it.
Since then, I have tried to restart it maybe 30-40 times and it is (almost) always the same symptom; relatively slow start (30 seconds or so) and then suddenly it freezes before coming to the login screen. Twice, the computer managed to start-up properly but shortly after the top part of the screen started to flimmer and the remainder went black and only a hard reset could get it out of its misery. But this has not happened since; now it just stalls during the grey screen start-up.
The iMac is a 24" from early 2008 (model EMM 2134) with 2,8GHZ and 4GB ram (apple supplied). About two years ago the hard drive failed and I had to replace it with a new one but this is the only modification I have applied. It runs OSX 10.9
To resolve the issue, I have been through various error checks that have not helped
- No visible or audible defects that I can discern
- The power supply inside the iMac does not seem to be broken (I opened and checked)
- Removing the two 2GB rams blocks one at a time does not change start-up behavior
- Booting from USB recovery or from an install DVD in the optical drive using either the C or D key during start-up does not help
- Starting in SAFE mode and enabling logging showed an SMCreadkeyaction ERROR
- Tried resetting the NVRAM (http://support.apple.com/kb/HT1379) and the SMC (http://support.apple.com/kb/HT3964.).
- Apple Disk Utility test and recovery (via another mac in firewire mode) turned up no fautls.
- Formatted HD and installed OSX 10.5 on the HD (via firewire on onther mac). Still no change.
I am a bit at a loss what could cause this problem; it does not seem to be harddisk related (though this cannot be ruled out), it does not seem to be the graphic card despite the occasional flimmer as most of the time, the screen looks just fine (including the firewire icon dancing around when I had in T mode); and if it had been the power supply would it not have had visible marks of burning or bulking condensators?
Qualified suggestions as to what might be wrong or how to keep testing would be greatly appreciated.
Thanks
Dan

An Apple repair station visit is in order.

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IMac freezes on start

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